Liquid nitrogen production

ABSTRACT

An improved process for liquid nitrogen production by cryogenic air separation using a distillation column system to enhance the product recovery.

BACKGROUND OF THE INVENTION

This invention concerns a new and efficient process for producing liquid nitrogen.

Liquid nitrogen is normally produced as a by-product of oxygen production. While there are sizable market demands for nitrogen by itself, in fields such as glass making, chemical inverting, electronics, and food preparation, these are end demands for the gas product, and the liquid form is merely a convenience for transportation and storage.

Large or stand alone demands for nitrogen gas are conventionally supplied by nitrogen gas generators, which often involve cryogenic distillation but produce no meaningful amounts of nitrogen in liquid form. When there is a large demand for the liquid form, without the simultaneous requirement of oxygen, nitrogen gas generators are typically coupled with a separate nitrogen liquefaction unit to fulfill this requirement.

SUMMARY OF THE INVENTION

It is a major object of the invention to provide an improved process for the direct production of liquid nitrogen, for example as a sole product in an integrated process, saving both power and capital. Basically, the improved process is enabled by use of systems containing distillation columns, operating in series at different pressure levels, to extract a higher yield of nitrogen per unit of compressed air feed processed, as will be seen. The process basically includes:

a) passing a portion of pressurized air feed through a cold expander and feeding a part of the air exhaust from said expander as a gaseous air stream into the bottom of a first column having a top condenser,

b) passing another portion of the pressurized air feed through the main heat exchanger for cooling and liquefying, passing said portion of air through a valve and feeding as a liquid air stream into the middle of the first column;

c) separating the said gaseous and said liquid air streams in the first column into a first liquid nitrogen stream and a first oxygen-enriched liquid stream;

d) feeding the first oxygen-enriched liquid stream into the middle of a second column having a bottom reboiler, in which the liquid is evaporated due to indirect heat exchange with the nitrogen vapor in the first column top condenser, and a top condenser;

e) separating the first oxygen-enriched liquid stream in the second column into a second liquid nitrogen stream and a second oxygen-enriched liquid stream;

f) feeding the second oxygen-enriched liquid stream into the middle of a third column having a bottom reboiler, in which the liquid is evaporated due to indirect heat exchange with the nitrogen vapor in the second column top condenser, and a top condenser, or into an upper reboiler;

g) separating the second oxygen-enriched liquid stream in the third column into a third liquid nitrogen stream and a third oxygen-enriched liquid stream, and feeding the third oxygen-enriched liquid stream into the upper reboiler;

h) evaporating the oxygen-enriched liquid stream in the upper reboiler due to indirect heat exchange with the nitrogen vapor in the third or second column top condenser;

i) removing the first liquid nitrogen stream from the first column and feeding it into the top of the third or second column;

j) removing the second liquid nitrogen stream from the second column and feeding it into the top of the third column or using it as a liquid nitrogen product;

k) removing the third liquid nitrogen stream from the third column and using it as a liquid nitrogen product;

l) removing the evaporated oxygen-enriched stream from the upper reboiler, warming it and removing from the process.

As will be seen, the cleaned and pressurized air feed is split in two streams, the first stream is passed through a warm expander, the second stream is further compressed in boosters by using expander power, previously cooled in a main heat exchanger and split in two portions, one portion of this air feed is passed through a cold expander and the other portion is further cooled and liquefied in the heat exchanger; using a tripe or double distillation columns system to enhance recovery of liquid nitrogen from air enabling substantial reduction in the feed air compressor, and absorber size and power.

A further object is to provide for use of these multiple distillation column systems to enhance recovery of nitrogen from air, and to permit use of an air recycle process to produce refrigeration. Significant reductions in main heat exchanger size and cost are enabled.

Yet another object is provision of a complete process including provision of distillation columns, condenser-reboilers, heat exchangers and compressors, operating as disclosed herein.

DRAWING DESCRIPTION

FIG. 1 is a schematic showing of a process for producing liquid nitrogen from air with a triple distillation column system;

FIG. 2 is a schematic showing a process for producing liquid nitrogen from air with a double distillation column system;

FIG. 2a is a schematic with a modified double distillation column system.

DETAILED DESCRIPTION

Referring to the schematic of FIG. 1, air feed at 10 is filtered in 101, and compressed at 102 to a pressure of 8 to 10 bara, cooled in 103, and after removal in adsorber 104 of water and carbon dioxide, the air is mixed with the recycle stream 22 removed from the main heat exchanger 113 and at 13 fed to the compressor 105, where it is further compressed to about 40 bara (+/−5), and cooled in 106.

A portion 16 (or all) of the compressed air stream 14 is then boosted in one or two compressors 107 and 109, driven by one or two turbo expanders 112 and 111, to a pressure between 70 and 90 bara at 18. The other portion 15 of the compressed air is fed in the warm turbo expander 111 and then to the heat exchanger 113 at 17.

The boosted air is then cooled in the heat exchanger 113, and a portion is fed at 19 to the cold turbo expander 112, the remainder 23 being further cooled and liquefied, then expanded in a valve 114 and fed to the middle of the first distillation column 115 as a liquid air stream.

The exhaust 20 from the cold turbo expander is split. One portion returns at 21 to provide cooling in the aforementioned heat exchanger 113 while the split remainder to fed at 25 into the bottom of the first distillation column 115 as a gaseous air stream. In that first column, operated about from 7.5 to 9 bara, the air is distilled into pure nitrogen (from 1.0 to 0.0001 mol % of O₂) at 26, and condensed in the top condenser 116, a portion of the condensate being removed at 27 as a first liquid nitrogen stream, cooled in the heat exchanger 121 and passed through a valve 122 into the top of the third column 119. The remainder returns at 28 to the first column as reflux.

The bottom liquid in the first column is rich in oxygen (24 to 26% O₂). This first oxygen-enriched liquid, removed at 29, is cooled in exchanger 121 and fed through a valve 123 into the middle of the second column (operated from about 5.0 to 6.5 bara), where it joins with liquid descending in this column. This liquid descends countercurrent to the vapor generated by the bottom reboiler 116.

The vapor ascending in the second column is progressively rectified until a pure nitrogen is achieved on the highest rectification stage. This nitrogen stream is condensed in the top condenser 118 and a portion removed at 31 and passed through valve 125 into the top of the third column 119, while the remainder descends as reflux.

The bottom liquid in the second column is richer yet in oxygen (32-33% of O₂), removed at 30 as a second oxygen-enriched liquid, throttled at 124 and fed into the middle of the third column 119 (operated at 3-3.6 bara), where it joins with liquid descending in this column. This liquid descends countercurrent to the vapor generated by bottom reboiler 118.

The vapor ascending in the third column is progressively rectified until a pure nitrogen is achieved on the highest rectification stage. This nitrogen stream is condensed in the top condenser 120 and a portion removed at 32 as a liquid nitrogen product while the remainder descends as reflux.

The bottom liquid in the third column is richer yet in oxygen (50-52% of O₂), removed at 33 as a third oxygen-enriched liquid, throttled in 126 to about one atmosphere and transferred to the upper reboiler 120.

A very small amount of the oxygen rich (approximately 78% O₂) liquid is removed at 34 from the upper reboiler 120 to guard against build up of a dangerous substances in that reboiler as contaminants.

The vapor exiting the upper reboiler (waste) moves through the sub-cooling heat exchanger 121 and enters at 36 the main heat exchanger 113 where the refrigeration is recovered. At the exit of the main exchanger some of the waste is vented, the rest being used at 38 to regenerate the adsorber 104 associated with water and carbon dioxide removal.

The example stream parameters of the process for producing liquid nitrogen at 88.9 K corresponding to the pressure in the third column 3.27 bara are shown in the Table 1. Using the process with the triple column system allows increasing the liquid nitrogen output (LIN) from 0.33 mol/mol of processed air (p.a.) for the existent process to 0.59 mol/mol p.a. (The processed air flow rate is equal to the feed air flow rate). The increase in liquid nitrogen recovery enables a 20% reduction in the specific power and substantial reduction in the feed air compressor and adsorber size and cost.

The liquid nitrogen product can be subcooled in 127 from the temperature about 86-89 K at 32 to the temperature about 79-81 K at 40 by evaporating a part of liquid nitrogen stream 41 in 128 at reduced pressure close to atmospheric. The evaporating a part of liquid air stream at reduced pressure for the preliminary subcooling can be also used.

Referring to the schematic of FIG. 2, air feed at 10 is filtered in 101, and compressed in 102 to a pressure of 5 to 7.5 bara, cooled in 103 and after removal in adsorber 104 of water and carbon dioxide, the air is mixed with the removed from the main heat exchanger 113 recycle stream 22 and at 13 fed to the compressor 105, where it is further compressed to about 30 bara (+/−5), and cooled in 106.

A portion 16 (or all) of the compressed air stream 14 is then boosted in one or two compressors 107 and 109, driven by one or two turbo expanders 112 and 111, to a pressure between 55 and 75 bara at 18. The other portion 15 of the compressed air is fed in the warm turbo expander 111 and then in the heat exchanger 113 at 17.

The boosted air is then cooled in the heat exchanger 113, and a portion 19 is fed to the cold turbo expander 112, the remainder 23 being further cooled and liquefied, then expanded in a valve 114 and fed in the middle of the first (lower) distillation column 115 as a liquid air stream.

The exhaust 20 from the cold turbo expander is split. One portion returns at 21 to provide cooling in aforementioned heat exchanger 113 while the split remainder 25 is passed through a throttling valve 119, wherein the pressure is decreased by up to 2 bar (for example, from 6.5 to 4.5 bara), and then at 33 fed into the bottom of the first (lower) column as a gaseous air stream.

In that first (lower) column, operated about from 4.5 to 6.5 bara, the air is distilled into pure nitrogen (from 1.0 to 0.0001 mol % of O₂) at 26, and condensed in the top condenser 116, a portion of the condensate being removed at 27, as a first liquid nitrogen stream, cooled in the heat exchanger 121 and passed through a valve 122 into the top of the second (upper) column. The remainder returns at 28 to the lower column as reflux.

The bottom liquid in the lower column is rich in oxygen (27 to 28% O₂). This first oxygen-enriched liquid removed at 29 is cooled in 121 and fed through a valve 123 into the middle of the upper column (operated from about 2.7 to 3.3 bara), where it joins with liquid descending in this column. This liquid descends countercurrent to the vapor generated by the bottom reboiler 116.

The vapor ascending in the upper column 117 is progressively rectified until a pure nitrogen is achieved on the highest rectification state. This nitrogen stream is condensed in the top condenser 118 and a portion removed at 32 as a liquid nitrogen product while reminder descends as reflux.

The bottom liquid in the upper column is richer yet in oxygen (43-45% of O₂), removed at 30 as a second oxygen-enriched liquid throttled in 124 to about one atmosphere and transferred to the upper reboiler 118.′

A very small amount of the oxygen rich (approximately 73% O₂) liquid is removed at 34 from the upper reboiler to guard against build up of a dangerous substances in that reboiler as contaminants.

The vapor exiting the upper reboiler (waste) moves through the sub-cooling heat exchanger 121 and enters at 36 the main heat exchanger 113 where the refrigeration is recovered. At the exit of the main exchanger some of the waste is vented, the rest being used at 38 to regenerate the adsorber 104 associated with water and carbon dioxide removal.

The example stream parameters of the process for producing liquid nitrogen an 88.1 K corresponding to the pressure 3.05 bara in the upper column are shown in the Table 2. Using the process with the double column system allows increasing the liquid nitrogen output (LIN) from 0.33 mol/mol of processed air (p.a.) for the existent process to 0.52 mol/mol p.a. The increase in liquid nitrogen recovery enables a 15% reduction in the specific power and substantial reduction in the feed air compressor and adsorber size and cost.

The part of the air exhaust from the cold expander is passed into the lower column through a throttling valve using for the expander exhaust pressure control and allowing also to additional increasing the liquid nitrogen recovery. For example, if the pressure in this valve is decreased by 2 bar (from 6.5 to 4.5 bara), the liquid nitrogen output (LIN) is increased by 0.021 mol/mol p.a., that is 3.8% (0.020/0.522=1.038).

The pressure of the liquid nitrogen product can be increased by using a liquid column. For example, if the difference in elevation between the top of the upper column and the place of withdrawal of the liquid nitrogen product is equal to 16 m, the pressure can be increased by 1.2 bar.

As previously noted, the liquid nitrogen product can be subcooled in 127 from the temperature about 86-89 K at 32 to the temperate about 79-81 K at 40 by evaporating a part of the liquid nitrogen stream 41 at reduced pressure close to atmospheric.

The example conditions of the process with the double column system (FIG. 2) for liquid nitrogen production at 81 K are illustrated in the Table 3. In this case the process with the double column system allows increasing the liquid nitrogen output (LIN) from 0.303 mol/mol of processed air (p.a.) for the existent process to 0.448 mol/mol p.a. The increase in liquid nitrogen recovery enables a 12% reduction in the specific power and also substantial reduction in the feed air compressor and adsorber size and cost.

Using the evaporating part of the liquid air stream at reduced pressure for the preliminary subcooling of liquid nitrogen product enables 13% reduction in the specific power as compared with the existent process.

The advantages of the air recycle are lower size (by 33%) of the main heat exchanger as compared with the nitrogen recycle.

The booster compressors 107 and 109 can operate in series as shown in FIG. 2, or in parallel. Series connection reduces the specific power by 1% compared to the parallel.

Feeding the liquid air stream into the lower column allows to increase the LIN output as compared with feeding this stream into the upper column. As is seen from the Table 4, LIN is increased by 4% (0.522/0.502=1.04). In addition, feeding the liquid air stream into the lower column allows reducing of the throttling valve and pipe size.

The liquid air stream fed into the lower column should be equal to at least 40% of the processed air.

The reflux ratio in the columns and correspondingly the number of trays makes a greater impact on the liquid nitrogen product yield and other parameters, that affect the energy and equipment costs. It is estimated that the optimal relationship between the reflux ratio and the minimum reflux ratio for the columns is approximately from 1.1. to 1.2, if the liquid nitrogen product contains about 0.01% of oxygen.

The waste gas removed from the main heat exchanger 113 contains from 38% O₂ (Table 3) to 43.4% O₂ (Table 2). For the purpose of decreasing the oxygen content in the regeneration gas, one or more trays can be added above the upper reboiler to provide two separate streams: a regeneration gas and a waste stream with increased oxygen content (Table 3).

This method of decreasing the oxygen content in the regeneration gas result in increasing the boiling temperature in the upper reboiler and the pressure in the columns and therefore leads to reducing the liquid nitrogen recovery. Another method in which the pressure in the columns is not increased is discussed below.

Referring to the schematic of FIG. 2a , the oxygen-enriched liquid from the upper column is removed at 30, throttled in 124 and transferred to the first upper reboiler 118, wherein this liquid is partially evaporated. The vapor that contains less oxygen is removed at 35, then heated in the exchangers 121 and 113 and used at 38 as a regeneration gas for the adsorber 104. The remainder liquid is removed from 118 at 39, throttled in 125 and fed into the second upper reboiler 120. The vapor exiting 120 at 42 is heated in the exchangers 121 and 113 and vented at 37 as a waste gas.

A very small amount of the oxygen rich (approx 80% O₂) liquid is removed at 34 from the second upper reboiler 120 to guard against build up of a dangerous substances in that reboiler as contaminants.

As is seen from the Table 5, the oxygen content in the regeneration gas is equal to 25%, that is much less than in case of using one upper reboiler (43.4% O₂), Table 2). The temperature difference in the first upper reboiler is equal to 3.74 K and in the second upper reboiler—1.17 K (Table 5), whereas the temperature difference in the case of using one upper reboiler is equal to 1.20 K (Table 2). The total surface of the first and second upper reboiler is less by 17% than the surface in case of using one upper reboiler. The part 25 of the air exhaust from the cold expander 112 can be passed into the lower column 115 through an additional expander 119 (FIG. 2a ) using for the receiving an additional refrigeration capacity. For example, if the pressure is expanded by 2 bar (from 6.5 to 4.5 tiara), the additional refrigeration capacity is equal to 2% of the total capacity, and the specific power can be decreased by 1.4% due to reducing the recycle air flow rate. It should be noted that the cold (and warm) expander exhaust pressure decrease is inexpedient, since it leads to a decrease in the efficiency of the recycle system.

ALTERNATIVE ARRANGEMENTS

a. The bottoms from the columns can be passed to the upper reboiler;

b. The liquid air stream can be fed to either column;

c. The first and second liquid nitrogen stream or the first liquid nitrogen stream can be used as a liquid nitrogen product;

e. The portions of the liquid nitrogen product removing from the distillation columns are passed through throttling valves into a liquid separator, from which the liquid is removed as a liquid nitrogen product at the temperature about 79-81 K and the vapor is passed through heat exchangers and removed from the process.

TABLE 1 The stream parameters of the process with the triple column system (FIG. 1) for producing liquid nitrogen at 88.9 K (example) Content Flow rate, Vapor of mol/mol Temperature, Pressure, mole oxygen, No p.a.* K bara fraction % mol 11 1.0 300.0 8.70 1.0 20.95 12 1.0 280.0 8.35 1.0 20.95 13 3.29 291.0 8.30 1.0 20.95 14 3.29 300.0 39.9 1.0 20.95 15 1.0 300.0 39.9 1.0 20.95 16 2.29 300.0 39.9 1.0 20.95 18 2.29 300.0 82.9 1.0 20.95 19 1.613 202.0 82.8 1.0 20.95 20 1.613 105.72 8.45 1.0 20.95 22 2.29 296.0 8.35 1.0 20.95 23 0.677 108.89 82.7 0.0 20.95 24 0.677 103.66 8.43 0.0572 20.95 25 0.323 105.72 8.45 1.0 20.95 27 0.1641 100.96 8.30 0.0 0.01 29 0.8359 104.06 8.45 0.0 25.06 30 0.6415 99.69 5.87 0.0 32.65 31 0.1943 95.93 5.72 0.0 0.01 32 0.5896 88.91 3.27 0.0 0.01 33 0.4104 94.70 3.42 0.0 51.03 34 0.0052 87.78 1.28 0.0 78.32 35 0.4052 87.78 1.28 1.0 50.68 *p.a.—processed air.

TABLE 2 The stream parameters of the process with the double column system (FIG. 2) for producing liquid nitrogen at 88.1 K (example) Content Flow rate, Vapor of mol/mol Temperature, Pressure, mole oxygen, No p.a.* K bara fraction % mol 11 1.0 300.0 6.40 1.0 20.95 12 1.0 280.0 6.05 1.0 20.95 13 3.03 291.0 6.0 1.0 20.95 14 3.03 300.0 31.0 1.0 20.95 15 0.88 300.0 31.0 1.0 20.95 16 2.15 300.0 31.0 1.0 20.95 18 2.15 300.0 64.2 1.0 20.95 19 1.566 194.0 64.1 1.0 20.95 20 1.566 101.05 6.15 0.995 20.95 22 2.03 296.0 6.05 1.0 20.95 23 0.584 103.94 64.0 0.0 20.95 24 0.584 96.79 5.18 0.0772 20.95 25 0.416 101.05 6.15 0.995 20.95 33 0.416 98.96 5.20 0.999 20.95 27 0.243 94.15 5.05 0.0 0.01 29 0.757 97.35 5.20 0.0 27.67 30 0.478 92.97 3.20 0.0 43.82 32 0.522 88.12 3.05 0.0 0.01 34 0.006 86.92 1.28 0.0 73.25 35 0.472 86.92 1.28 1.0 43.45 *p.a.—processed air.

TABLE 3 The performance of the process with the double column system (FIG. 2) for producing liquid nitrogen at 81 K (example) # of case 1.2A Recycle Air Type of scheme DCU Feed air compressor flow rate, Nm{circumflex over ( )}3/h 6920 suction pressure, bara 0.99 discharge pressure, bara 6.40 Recycle compressor flow rate, Nm{circumflex over ( )}3/h 19151 Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 2.7677 suction pressure, bara 6.05 discharge pressure, bara 31.03 Main exchanger temperature, K middle pressure air inlet 300 temperature difference, K warm end 3.7 minimum warm section 2.5 cold section 1.6 U*A, kW/K 282 ‘Warm’ expander inlet pressure, bara 53.2 outlet pressure, bara 6.11 inlet temperature, K 300 outlet temperature, K 177.0 isentropic efficiency 0.86 flow rate, Nm{circumflex over ( )}3/h 4860 Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 0.7024 ‘Cold’ expander inlet pressure, bara 49.4 outlet pressure, bara 6.15 inlet temperature, K 177.4 outlet temperature, K 101.1 vapor mole fraction 0.993 isentropic efficiency 0.88 flow rate, Nm{circumflex over ( )}3/h 10786 Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 1.5588 Lower column Pressure, top, bara 5.95 Vapor flow rate, Nm{circumflex over ( )}3/h 3415 Concentration liquid nitrogen, ppm O2 3 kettle liquid, % mol O2 38.0 Number of theoretical trays 46 Condenser-reboiler temperature difference, K 2.7 Upper column Pressure, top, bara 3.2 Vapor flow rate, Nm{circumflex over ( )}3/h 3688 Concentration liquid nitrogen, ppm O2 3 kettle liquid, % mol O2 46.1 Number of theoretical trays 40 Condenser-reboiler temperature difference, K 1.5 Liquid Nitrogen product LIN output, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 0.448 LIN capacity, Nm{circumflex over ( )}3/h 3100 Temperature, K 81 Pressure, bara 3.2 Regeneration gas and waste gas flow rate, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 0.552 middle concentration, % mol O2 37.9 Regeneration gas flow rate, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 0.25 concentration, % mol O2 21.0 Waste gas flow rate, Nm{circumflex over ( )}3/Nm{circumflex over ( )}3 p.a. 0.302 concentration, % mol O2 51.9 p.a.—processed air.

TABLE 4 The performance of the double column system at feeding the liquid air stream into the lower or upper column (example) # of case 4.1 4.2 4.3 Feeding the liquid air lower upper upper stream into column column column Feeding the bottom upper upper upper liquid from the lower column column reboiler column into Liquid air stream, 0.584 0.563 0.563 mol/mol p.a. Lower column Pressure (top), bara 3.05 3.05 3.05 Concentration, % mol O2 liquid nitrogen 0.01 0.01 0.01 kettle liquid 27.67 40.13 40.13 LIN output, mol/mol 0.243 0.2089 0.2089 p.a. Number of theoretical 36 36 36 trays (NTT) section 1 32 36 36 section 2 4 Upper column Pressure (top), bara 5.05 5.05 5.05 Concentration, % mol O2 liquid nitrogen 0.01 0.01 0.01 kettle liquid 43.82 42 43.8 LIN output, mol/mol 0.522 0.5013 0.5027 p.a. Number of theoretical 36 36 36 trays (NTT) section 1 32 30 32 section 2 4 2 4 section 3 4

TABLE 5 The stream parameters of the process in the first and second reboilers (FIG. 2a) (example) Content Flow rate, Vapor of mol/mol Temperature, Pressure, mole oxygen, No p.a.* K bara fraction % mol 30 0.478 92.97 3.20 0.0 43.82 32 0.522 88.12 3.05 0.0 0.01 34 0.006 86.95 1.13 0.0 80.71 35 0.17 84.38 1.28 1.0 24.86 39 0.308 84.38 1.28 0.0 54.27 42 0.302 86.95 1.13 1.0 53.73 *p.a.—processed air. 

1. A system for liquid nitrogen production by cryogenic air separation comprising: a) a first column operating to distill a gaseous air feed stream and a liquid air feed stream into a liquid nitrogen stream and an oxygen-enriched liquid stream; b) second and third columns, at least one operating to distill the oxygen-enriched liquid stream into a liquid nitrogen stream and an oxygen-enriched liquid stream with enhanced content of oxygen; c) three condenser-reboilers associated with the respective first, second and third columns and operating to effect indirect heat exchange between the nitrogen vapor at the top of the column and the oxygen-enriched liquid at the bottom of the higher column or columns; d) the upper condenser-reboiler operating to effect indirect heat exchange between the nitrogen vapor at the top of the third or second column and the oxygen-enriched liquid in an upper reboiler; e) means for passing a first portion of the pressurized air feed through a cold expander and feeding a part of the air exhaust from said expander as a gaseous air stream into the bottom of the first column; f) means for passing a second portion of the pressurized air feed through a main heat exchanger for cooling and liquefying, passing the second portion of air through a throttling valve and feeding as a liquid air stream into the middle of the first column; h) means for passing the oxygen-enriched liquid stream from the first column to the second column, from the second column to the third column or to the upper reboiler, and from the third column to the upper reboiler; i) means for removing the liquid nitrogen in three streams as liquid nitrogen products; j) wherein the three columns have a serial arrangement whereby oxygen enriched liquid is introduced successively from the high pressure first column to the intermediate pressure second column, and then from the intermediate pressure column to the low pressure third column.
 2. The system of claim 1 further comprising in combination: a) means for subcooling the liquid nitrogen product by evaporating a part of liquid nitrogen stream at reduced pressure; b) means for subcooling the liquid nitrogen product by passing the liquid nitrogen streams removed from the distillation columns through throttling valves and into a liquid separator; c) one throttling valve passing the air exhaust from the cold expander to the first column; d) an additional expander at the stream passing of the air exhaust from the cold expander to the first column; e) one or more trays above the upper reboiler to provide two separate streams comprising a regeneration gas with decreased oxygen content, and a waste gas; f) two upper reboilers operated in series to provide two separate streams; a regeneration gas with decreased oxygen content, and a waste gas.
 3. A method of separating liquid nitrogen products from air comprising: a) providing first and second distillation columns; b) passing a portion of pressurized air feed through an expander; c) passing part of the exhaust from said expander to the first distillation column; d) distilling air in the first distillation column to produce 99 to 99.99% pure nitrogen gas, and condensing said produced nitrogen gas in a first reboiler to produce a condensed nitrogen product; d) passing bottoms from the first distillation column, after cooling, to the second distillation column to join with descending reflux in that column; e) wherein vapor ascending in the second distillation column is progressively rectified until pure N₂ is produced, removed and condensed in a second reboiler, as product nitrogen.
 4. The method of claim 3 wherein the expander is a turbo expander that produces said air exhaust.
 5. The method of claim 3 wherein said turbo expander drives at least one compressor which pressurizes said air feed.
 6. The method of claim 3 further comprising: removing H₂O and CO₂ from the air feed between stages of compression.
 7. The method of claim 6 wherein said part of the air exhaust from the expander is fed as vapor to the first distillation column.
 8. The method of claim 6 further comprising: providing and operating a primary heat exchanger configured to cool the air flow to the turbo expander and to operate the turbo expander.
 9. The method of claim 3 wherein another part of the pressurized air feed is passed through a valve operating to form cooled liquid air.
 10. The method of claim 3 wherein vapor exiting a second reboiler associated with the second distillation column is passed through a secondary heat exchanger and then to a primary heat exchanger for recovery of refrigeration.
 11. A system for separating liquid nitrogen produced from air comprising: a) first and second distillation columns; b) means for passing a portion of pressurized air feed through an expander and passing part of air exhaust from said expander to the first distillation column, and for passing a liquefied portion of the air feed to the first distillation column; c) the first distillation column configured to distill air in the first column to produce 99 to 99.99% pure nitrogen gas, and to condense said produced nitrogen gas in a first reboiler to produce a condensed nitrogen product; d) means for passing an oxygen enriched liquid stream from the first distillation column, after cooling, to the second distillation column to join with descending reflux in the second distillation column, and for passing an oxygen enriched liquid stream from the second distillation column to a third distillation column, liquid nitrogen being withdrawn in separate streams from the three distillation columns; e) wherein the three distillation columns have a serial arrangement whereby oxygen enriched liquid is introduced successively from the first distillation column at a high pressure to the second distillation column at an intermediate pressure, and then from the second distillation column to the third distillation column at a low pressure.
 12. The system of claim 11 wherein the expander is a turbo expander producing said air exhaust, said expander driving at least one compressor raising the pressure of the air feed.
 13. The system of claim 12 including means feeding part of the air exhaust from the expander in vapor state to the first distillation column.
 14. The system of claim 11 including means for flowing part of the pressurized air feed through a valve operating to form cooled liquid air.
 15. The system of claim 11 including means for flowing vapor exiting a first reboiler associated with the first distillation column to a secondary heat exchanger and then to a primary heat exchanger for recovery of refrigeration.
 16. The combination of claim 11 wherein the first distillation column is a high pressure column and the second distillation column is a low pressure column. 